THE GIST

The world's last known surviving non-avian dinosaur was a Triceratops from Montana's Hell Creek Formation.

The discovery suggests dinosaurs did not gradually die out before 65 million years ago, but that they went suddenly extinct.

Hoofed mammals and rodent-like species were among the animals that flourished after the extinction event.

A Triceratops may have been the last dinosaur standing, according to a new study that determined a fossil from Montana's Hell Creek Formation is "the youngest dinosaur known to science."

The Triceratops, described in the latest Royal Society Biology Letters, dates to 65 million years ago, the critical period of time associated with the Cretaceous-Tertiary (K-T) extinction event that wiped out all non-avian dinosaurs and many other animals and plants.

Since this rhinoceros-looking, three-horned dinosaur lived so close to the mass extinction moment, it could negate an earlier theory that dinosaurs gradually died out before 65 million years ago.

"Our paper suggests that dinosaurs did not go extinct prior to the impact," lead author Tyler Lyson told Discovery News. "The fact that this dinosaur is so close to the K-T boundary lends support to the idea that they went extinct as a result of a meteorite impact."

Lyson, a researcher in Yale University's Department of Geology and Geophysics, and his team discovered the remains of the Triceratops, including its over 1.5-foot-long horn, just 5 inches below the pollen-calibrated K-T boundary at Camel Butte, a hill at the Hell Creek Formation in southeastern Montana.

By studying the region's geological layers, the scientists can see how dinosaurs suddenly disappeared after the catastrophic event, which Lyson and many other experts believe was a meteorite strike that directly hit Earth at Mexico's Yucatan Peninsula.

Lyson said that "we don't fully understand the kill mechanism," but other researchers "have a proposed a nuclear winter, while others have proposed a thermal pulse."

The prior theory that dinosaurs gradually died out before 65 million years ago was often based on what is known as the "3-meter gap," which referred to an apparent geological zone devoid of dinosaur fossils before the K-T event.